1. Field of the Invention
The present invention relates to a roller mill for grinding particulate material such as cement raw materials, cement clinker and similar materials, said roller mill comprising a substantially horizontal grinding table and a set of rollers rotatable about a vertical shaft, said set of rollers comprising a number of rollers rotating about separate roller shafts which are connected to the vertical shaft via a hinged connection which comprises a bearing shell and a bearing journal resting therein, and allowing a free circular movement of the roller in upward and downward direction in a plane comprising the centreline of the roller shaft, said set of rollers being configured for interactive operation with the grinding table.
2. Description of Related Art
Roller mills of the aforementioned kind are well known. In the known roller mills the hinged connection which connects the roller shaft with the vertical shaft typically consists of a traditional slide bearing which may be lubricated using a suitable lubricant. The function of the hinged connection is to ensure that, independently of one another, the rollers will be able to follow the height variations occurring in the layer of material deposited on the grinding table during the operation of the mill. In terms of direction and magnitude, the force to be absorbed by the hinged connection is mainly constant relative to the bearing journal, the angular motion of which is produced by the variations in the thickness of the material layer, typically ranging within an interval of ±0.5 to 5 degrees. The frequency of the angular motion of the bearing journal will typically be within the interval of 0.5 to 1 Hz. When this known roller mill is used for grinding particulate material such as cement raw materials, cement clinker and similar materials, the hinged connection will be subjected to a relatively high pressure which subject to friction between the parts of the hinged connection will give rise to a detrimental generation of heat which cannot readily be reduced by means of a lubricant since the unilateral constant application of pressure and the very small angular motions back and forth of the bearing journal may preclude the lubricant from being drawn into the zone of loading. Of other commercially available bearing types may be mentioned a hydrodynamic slide bearing which cannot be used since the bearing journal is not continuously turned and hence does not build-up a hydrodynamic lubricating film, a hydrostatic radial bearing which from a technical viewpoint is an ideal bearing solution ensuring a full level of contact whether in static operation or subject to rotation, but being too complicated, sensitive and costly, and a rolling bearing which is not suitable since the small angular motions will prevent the journals from drawing lubricant into the zone of loading. So, the conclusion to be drawn is that none of the traditional, commercial bearing types have the characteristics required to handle the load situation described without entailing significant disadvantages.